In preparing a magnetic coating composition by mixing a resin binder with magnetic powder, for the manufacture of a magnetic recording medium, it is necessary to improve the mutual adhesion of the magnetic powder and the binder and also the dispersibility of the powder into the binder so that the product can attain better magnetic and electrical properties. To this end a variety of surface active agents are in use.
Attempts have been made to employ as dispersants for magnetic powder, e.g., higher aliphatic amines, higher fatty acids, phosphoric esters of higher alcohols, such as polyoxyethylene phosphate alkyl ethers, esters of higher fatty acids and sorbitol, sodium alkylbenzenesulfonate, betaine type nonionic surface active agents, and the like. The magnetic coatings of recording media, obtained by adding those surface active agents to magnetic coating compositions, exhibit uniform dispersion and high filling rates of the magnetic powder, good orientation where the powder is in the form of acicular particles, and other merits. On the other hand, the addition of such a surface active agent increases the frictional force that the coating surface develops as it slides past the magnetic head or the tape-control guide pins, or between the rotary cylinders, of a magnetic recording-playback system, or even reduces the strength of the magnetic coating or film. This often causes troubles with cassette tapes, video tapes, etc., such as stall with the tape twining round, or caught in, the head recorder system, problems of running tape, such as wow, flutter, jitter, and skew, and other troubles that stem from the weakened magnetic coating, including a drop in output, head clogging, frequent dropouts, and a decrease in reproduction output due to the deposition of the magnetic coating components on the parts forming the head gap of the system.
These troubles with the head recorder system arise particularly from operation under hot and humid conditions or at low temperatures. This is generally ascribed to the fact that the surface active agent of a low molecular weight that contributes to the uniform dispersion of the magnetic powder passes into the magnetic coating surface or unnecessarily plasticizes the binder, the other component of the coating film.
Thus, the greater the proportion of the surface active agent added for more uniform dispersion of fine magnetic particles, the more profound and the more widespread the aforementioned difficulties will be. The present invention is aimed at resolving the contradiction between the improvement in dispersibility of magnetic powder and the deterioration in running quality or wear resistance by the addition of a conventional surface active agent.